1. Field of the Invention
The present invention relates to a method for making through-holes in a silicon substrate and an ink-jet printer head fabricated by the method. More particularly, the present invention aims at improving the formation yield of the through-holes.
2. Description of the Related Art
Recently, intensive research has been conducted regarding methods for making through-holes in silicon substrates by isotropic or anisotropic etching, and application of the methods to devices.
In Japanese Patent Laid-Open No. 10-181032, the applicant of the present invention discloses a method for making a through-hole, in which a sacrificial layer is formed on a silicon substrate before making the through-hole, and thereby, the size of the through-hole is controlled and the positional accuracy of the through-hole is improved. Furthermore, as an improvement of the method disclosed in Japanese Patent Laid-Open No. 10-181032, the applicant of the present invention also discloses a method in which a protective layer is disposed on the sacrificial layer to improve the formation yield of through-holes, or a method in which the sacrificial layer is embedded in the silicon substrate, and thereby, the size of the through-hole is further controlled and the positional accuracy of the through-hole is further improved. In Japanese Patent Laid-Open No. 6-347830, the applicant of the present invention discloses that a silicon nitride film formed by low-pressure vapor deposition (LP-SiN) is effective as an etching stop layer in the through-hole formation process. In Japanese Patent Laid-Open No. 9-11479, the applicant of the present invention also discloses a method in which a through-hole is made in a silicon substrate, and the through-hole is used as an ink supply port of an ink-jet head.
However, although the positional accuracy of the through-hole is greatly improved by the sacrificial layer disposed on the silicon substrate, cracks may occur in the etching stop layer when the hole penetrates the silicon substrate, resulting in defects, such as intrusion of the etchant into the surface of the substrate.
FIGS. 4A to 4E are sectional views showing steps in a conventional method for making a through-hole using a sacrificial layer. Referring to FIG. 4A, a sacrificial layer 402 composed of polycrystalline silicon (hereinafter referred to as poly-Si) and an etching stop layer 403 are disposed on a first surface of a silicon substrate 401, and an etching mask layer 404 is disposed on a second surface of the substrate 401.
In this method, as shown in FIG. 4B, a through-hole is made from the second surface to reach the inside of the sacrificial layer 402. When the hole penetrates the substrate 401, the sacrificial layer 402 is immediately dissolved in the etchant, and anisotropic etching starts from the edge of the sacrificial layer 402. Finally, the through-hole has a shape shown in FIG. 4C.
In anisotropic etching of the {100} plane of a silicon substrate, in theory, etching stops at the {111} plane, and a through-hole is made at an angle of 54.7° relative to the plane of the substrate. The size and position of the through-hole are uniformly set. In practice, in many cases, due to uneven thickness of the silicon substrate and crystal defects of the silicon substrate, the size and position of the through-hole vary to some extent. In particular, when a through-hole is made after a semiconductor element is preliminarily embedded in a silicon substrate, in some cases, the crystal defects are increased by thermal hysteresis in the semiconductor formation process, resulting in an increase in variations in the size and position of the through-hole.
In the method using the sacrificial layer, since the opening shape and the position of the through-hole can be controlled by the placement of the sacrificial layer, fabrication can be performed more accurately. However, in the method described above, since the etching stop layer is disposed on the sacrificial layer, as shown in FIG. 4D, coverage at the corner is insufficient, and cracks occur more easily, resulting in a decrease in the yield. If the etchant intrudes into the surface of the substrate due to the cracks, damage is caused because, in order to save time for etching, the silicon substrate is usually etched using a strong alkali solution, such as a tetramethylammonium hydroxide (TMAH) or potassium hydroxide (KOH) solution, at a temperature of 80° C. or more.
In the method in which the sacrificial layer is embedded in the silicon substrate, the number of fabrication steps is remarkably increased because of restrictions on masks in the presence of the embedded section.
In order to eliminate the defects, a protective film 410 may be formed above the corner (refer to FIG. 4E) so that the etchant is prevented from intruding into the surface of the substrate even if cracks occur at the corner. In such a case, however, the number of fabrication steps increases because a step of forming the protective layer is included.